"...difference between the 3 bar and the 5 bar oil pressure gauges..."

2 bar.

"Bar" stands for "barometric." In essence, 1 bar equals atmospheric pressure at sea level, or 14.7 psi absolute. Considering it as 15 psi is close enough.

Therefore, your 3 bar gauge measures about 45 psi and a 5 bar gauge would measure 75 psi max.

My '82 turbo has the stock 3 bar gauge. I've considered upgrading, and even have a handful of 5-bar gauges (and senders) but decided to stay with the 3-bar gauge.

Why?

Because the 3-bar gauge will give me a more accurate and expanded mesaurement of lower oil pressure at hot idle than will a 5-bar gauge. Frankly, with high oil pressure, I don't really care if it's 4.5 or 4.7 bar. But I DO care if the hot idle pressure drops from 2.1 to 1.9 to 1.7 bar.

The 3-bar gauge expands the more critical low-pressure range.

Keep in mind that the 3-bar gauge uses one sender and the 5-bar gauge uses a different sender.
--
Don Foster (near Cape Cod, MA)

A bar is just a measurement of Force per unit area. (akin to PSI) I think a bar is between 15-17 PSI, the conversion can be found on the web.

My 5 bar gauge is often pegged, and once the car warms up, the gauge reads about 2.5 bar at idle.

The 5 bar and 3 bar gauges do require different senders.